Exhaust Line for a Heat Engine

ABSTRACT

The exhaust line for a heat engine comprises, between an exhaust inlet and an exhaust outlet, a nitrogen oxide trap which is fitted on a filtering portion. It comprises, between the nitrogen oxide trap and the exhaust inlet, a branching portion which is capable of branching the exhaust gases away from the nitrogen oxide trap and switching means for the exhaust gases in order to cause them to flow through the filtering portion or branching portion.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/915,439, filed Jun. 23, 2008, which is the U.S. national phase ofPCT/FR2006/001212, filed May 29, 2006, which claims priority to FR9595264, filed May 25, 2005.

TECHNICAL FIELD

The present invention relates to an exhaust line for a heat engine, ofthe type comprising, between an exhaust inlet and an exhaust outlet, anitrogen oxide trap which is fitted on a filtering portion.

BACKGROUND

Currently, specific motor vehicles comprise internal combustion engineswhich operate principally with a mixture of fuel and oxidant which isreferred to as lean. A mixture of this type is characterized by anexcess of oxidant relative to fuel, the mixture which supplies theengine being substoichiometric in terms of fuel. Conversely, an enginewhich operates under conditions which are superstoichiometric in termsof fuel is said to be supplied with a rich mixture.

Engines of this type are used owing to their reduced fuel consumption.However, they operate under conditions such that significant quantitiesof nitrogen-containing derivatives such as nitrogen oxides are produced.Nitrogen oxides of this type, generally referred to as NOx, are harmfulto the environment and it is desirable to eliminate them.

To this end, the exhaust line of the motor vehicle is equipped withcatalytic depollution means and a nitrogen oxide trap which is generallyreferred to using the acronym “NOx trap”.

As known per se, the nitrogen oxide trap adsorbs the nitrogen oxides onspecific catalytic elements when the engine operates with a leanmixture. When the adsorption limit of the nitrogen oxide trap isreached, they are regenerated. To this end, the engine is controlled soas to operate with a rich mixture for a few seconds. A rich mixture ischaracterized by superstoichiometric conditions in terms of fuel, themixture comprising an excess of fuel relative to the oxidant introduced.

Nitrogen oxide traps operate in a satisfactory manner in a range oftemperatures of between 300° C. and 600° C. They are heated by theexhaust gases themselves. Therefore, the nitrogen oxide traps arearranged, along the length of the exhaust line, as close as possible tothe engine.

However, this position on the exhaust line renders the nitrogen oxidetraps incompatible with engines whose maximum operating temperatures mayreach 900° C. Above 600° C., the catalytic material present in thenitrogen oxide trap is destroyed.

It was envisaged to develop catalytic materials which withstand veryhigh temperatures but the results are not very satisfactory. It was alsoenvisaged to arrange, upstream of the nitrogen oxide trap, aheat-exchanger which allows the temperature of the exhaust gases to bereduced before they pass through the nitrogen oxide trap. However, thecost of this solution is very high.

The object of the invention is to provide an exhaust line for a heatengine which comprises a nitrogen oxide trap whilst being compatiblewith a heat engine for which the outlet temperatures of the exhaustgases are greater than the temperatures which the catalytic elements ofthe nitrogen oxide trap are capable of withstanding.

SUMMARY

To this end, the invention relates to an exhaust line for a heat engineof the above-mentioned type, characterized in that it comprises, betweenthe nitrogen oxide trap and the exhaust inlet, a branching portion whichis capable of branching the exhaust gases away from the nitrogen oxidetrap and switching means for the exhaust gases in order to cause them toflow through the filtering portion or branching portion, and in that itcomprises a three-way catalyzer which is arranged between the inlet andthe upstream portion of the filtering portion and the branching portion.

According to specific embodiments, the exhaust line comprises one ormore of the following features:

-   -   the switching means comprise means for blocking the filtering        portion immediately downstream of the nitrogen oxide trap;    -   the switching means comprise a three-way valve which connects in        parallel the branching portion and the filtering portion to the        remainder of the line;    -   the exhaust line comprises a fork which is arranged between the        inlet and, on the one hand, the upstream portion of the        filtering portion and, on the other hand, the upstream portion        of the branching portion, and the three-way valve is arranged        downstream of the filtering portion and the branching portion        and is capable of selectively connecting them to a common        downstream portion of the exhaust line;    -   the exhaust line comprises, on the one hand, at least one sensor        selected from the group comprising a sensor for sensing the        position of at least one valve, a temperature sensor and a gas        composition sensor and, on the other hand, means for diagnosing        the operating state of the switching means based on the or each        sensor;    -   the exhaust line comprises means for controlling the switching        means, which control means comprise means for acquiring the        outlet temperature of the exhaust gases from the engine, and the        switching means are capable of generally branching the gases off        towards the branching portion when the temperature of the gases        is greater than 600° C.;    -   the exhaust line comprises means for controlling the switching        means, which control means comprise means for acquiring the        stoichiometric operating conditions of the engine, and the        switching means are capable of generally branching the gases off        towards the branching portion when the engine operates in a        state which is superstoichiometric in terms of fuel;    -   the control means are capable of periodically controlling the        switching means so as to temporarily cause the exhaust gases to        flow through the filtering portion during the phases in which        the exhaust gases generally flow through the branching portion;        and    -   the exhaust line comprises means for controlling the switching        means, which control means comprise means for acquiring the        future occurrence of a phase for operating the engine with a        lean mixture when it operates with a rich mixture, and the        control means are capable of controlling the switching means so        as to branch the gases off towards the filtering portion when        such an operating phase of the engine with a lean mixture is        detected.

The invention also relates to a propulsion assembly which comprises anengine which generally operates with a lean mixture and an exhaust lineas described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from a reading of the followingdescription, given purely by way of example and with reference to thedrawings, in which:

FIG. 1 is a perspective view of an exhaust line according to theinvention;

FIG. 2 is a schematic view of the exhaust line according to theinvention with the valve in a first position which allows flow throughthe nitrogen oxide trap;

FIG. 3 is a flow chart which sets out in detail the control principle ofthe valve of the exhaust lines of FIGS. 1 and 2; and

FIGS. 4A, 4B, 4C are flow charts which set out in detail the diagnosticalgorithms implemented in the exhaust line according to the invention.

DETAILED DESCRIPTION

The exhaust line 10 illustrated in FIGS. 1 and 2 is intended to beconnected at the outlet of a heat engine which generally operates with alean mixture.

The exhaust line thus comprises an inlet 12 which is capable of beingconnected to the exhaust cylinder head of the engine and an outlet 14for discharging the exhaust gases into the atmosphere.

The inlet 12 is formed by a manifold 16. It is connected to the inlet ofa three-way catalytic purification element which is known per se andwhich is generally referred to below as a three-way catalyzer.

Downstream of the catalyzer 18, the line comprises a tube 20 which isextended with a fork 22 which simultaneously supplies a filteringportion 24 and a branching portion 26 which are arranged in parallel.The filtering portion 24 comprises a nitrogen oxide trap 28.

The nitrogen oxide trap 28 is mounted as close as possible to the outletof the engine along the length of the exhaust line so as to be broughtrapidly to temperature. In specific instances, the portion of theexhaust line between the nitrogen oxide trap and the outlet of theengine is thermally insulated.

The branching portion 26 has no processing means at all. The twoportions 24 and 26 are connected to each other in a downstream directionso as to open through an outlet tube 30 via a three-way valve 32 whichis controlled by an actuator element 34. The outlet 14 is formed at theend of the tube 30.

As illustrated in FIG. 2, the three-way valve comprises a flap 33 whichcan be moved between a first position in which the filtering portion 24is connected to the outlet portion 30, the branching portion 24 beinginsulated from the outlet portion 30, and a second position in which thebranching portion 24 is connected to the outlet portion 30, thefiltering portion 24 being insulated from the outlet portion.

The actuator 34 is connected to a control unit 36 for the controlthereof, which unit 36 receives information specific to the operation ofthe vehicle. In particular, a temperature sensor 38 is providedimmediately downstream of the manifold 16. This is connected to thecontrol unit 36.

Furthermore, the control unit 36 is connected to the unit 40 forcontrolling the operation of the engine which controls in particular theoperating phases of the engine with a rich mixture or lean mixture.

The units 36 and 40 are capable of exchanging information and controlcommands. In particular, the unit 36 is capable of acting on the unit 40so that it reduces the performance levels of the engine, so that itchanges supply mode to a rich mixture or lean mixture or so that itbrings about a regeneration phase.

Conversely, the unit 40 is capable of informing the unit 36 of thefuture initiation of a regeneration phase or of the supply mode of richmixture or lean mixture being used.

Furthermore, the control unit 36 integrates a module 42 for diagnosingthe operating state of the exhaust line.

According to a first embodiment, a sensor 44 for determining theposition of the valve 32 is placed on the exhaust line and connected tothe module 42.

In a variant, a temperature sensor 46 is arranged, in addition theretoor in place thereof, on the filtering portion 24 upstream of thenitrogen oxide trap 28.

According to still another variant, a sensor 48 for sensing gascomposition, for example, a sensor for sensing the level of nitrogenoxides, is placed in addition thereto or in place thereof downstream ofthe valve 32 along the outlet portion 30.

The control unit 36 is suitable for implementing an algorithm forcontrolling the valve 32 and the control unit 40 of the engine asillustrated in FIG. 3.

During a start phase of the engine, at step 100, the valve 32 ispositioned, at step 102, in a position for blocking the branchingportion 26 and for opening the filtering portion 24. In this manner,regardless of whether the engine is in supply mode with a rich mixtureor lean mixture, the nitrogen oxide trap 28 is heated under the actionof the flow of the exhaust gases.

At step 104, it is determined whether the engine operates with a richmixture. If this is not the case, it is determined, at step 106, whethera regeneration of the nitrogen oxide trap 28 is required. If this is thecase, the unit 36 controls the unit 40 so as to control the enginetemporarily with a rich mixture at step 106 in order to allow desorptionand a reaction of the nitrogen oxides, as known per se.

If, at step 106, a regeneration is not necessary, or following theregeneration operation, step 104 and subsequent steps are implementedagain.

If at step 104, the engine operates with a rich mixture, it isdetermined, at step 110, whether the temperature of the gases measuredby the sensor 38 at the outlet of the engine is greater than 600° C. Ifthis is not the case, step 104 and subsequent steps are implementedagain.

If this temperature is greater than 600° C., the control unit 36controls, at step 112, the switching of the valve 32, so that thefiltering portion 24 is blocked and the branching portion 26 is, incontrast, placed in communication with the outlet portion 30.

At step 114, it is determined, from the processor 40, whether a changeto a lean mixture is anticipated in the short term. If this is not thecase, a delay time is initiated at step 116 and, as long as this delaytime has not elapsed, step 104 and subsequent steps are implementedagain.

Once this delay time has elapsed, the valve 32 is temporarily switchedso that, for a shortened period of time, the exhaust gases flow throughthe nitrogen oxide trap 28, thus allowing it to increase in temperatureso that it is maintained in a range of temperatures between 300° C. and600° C.

Following this temporary switching phase, the valve 32 is returned intoposition at step 118 so that the gases flow through the branchingportion 24 and step 104 and subsequent steps are implemented again.

If, at step 114, the control unit 36 detects an imminent requirement tochange to a lean mixture to control the engine, the valve 32 is switchedat step 120 so that the gases flow only through the nitrogen oxide trap28 in order to increase the temperature thereof, in anticipation of thechange to a lean mixture. Following a predetermined period of time, thevalve is retained in position 32 and the mixture supplying the engine ismodified in order to allow control with a lean mixture at step 122.

It will be appreciated that, with an exhaust line of this type, thenitrogen oxide trap never operates at a temperature greater than 600° C.and is constantly kept at a temperature between 300° C. and 600° C.,allowing the exhaust gases to be processed at any time when the engineis supplied with lean mixture.

FIGS. 4A, 4B and 4C illustrate diagnostic phases which are permanentlyimplemented by the diagnostic unit 42, it being understood that one ormore of the phases illustrated in these Figures can be implementedjointly.

In FIG. 4A, the test 140 determines from the sensor 44 whether the valve32 is blocked. If this is the case, the control unit 36 controls, atstep 142, the unit 40 in order to adapt control of the engine, so thatit does not produce nitrogen oxides and/or maintains the temperature ofthe gases at less than 600° C.

In the presence of a temperature sensor 46, at step 150 illustrated inFIG. 4B, it is determined whether the temperature upstream of thenitrogen oxide trap is greater than 600° C. If this is the case, theperformance levels of the engine are reduced, at step 152.

If a sensor 48 for sensing the composition of the gases is provided atthe outlet, it is verified, at step 160, whether the content of nitrogenoxides is greater than a reference value V_(ref). If this is the case,the unit 36 controls the unit 40 so as to operate the engine with a richmixture, at step 162. Following this, the valve 32 is switched at step164 into the position thereof so that the exhaust gases flow only in thebranching portion 24.

Generally, in the case of a malfunction as set out in FIGS. 4A, 4B and4C, the control of the engine is adapted for operation in downgradedmode, that is to say, operation with a rich mixture and/or limitation ofthe outlet temperature of the gases. Furthermore, the driver is informedby an indicator light on the dashboard of the vehicle.

In this manner, even in the event of a malfunction in the exhaust line,the nitrogen oxide trap is preserved and the exhaust gases released intothe atmosphere have a low content of nitrogen oxide, even if theperformance levels of the engine, in terms of consumption or power, arereduced.

In a variant, the three-way valve 32 and the fork 22 are interchanged sothat the three-way valve is arranged upstream of the filtering andbranching portions.

In a variant of one or other of the embodiments, the three-way valve isreplaced by two separate two-way valves, one of the valves beinginstalled on the branching portion, the other valve being installed onthe filtering portion.

Although an embodiment of this invention has been disclosed, a worker ofordinary skill in this art would recognize that certain modificationswould come within the scope of this disclosure. For that reason, thefollowing claims should be studied to determine the true scope andcontent of this disclosure.

1. An exhaust line for a heat engine, comprising: between an exhaustinlet and an exhaust outlet, a nitrogen oxide trap which is fitted on afiltering portion; between the nitrogen oxide trap and the exhaustinlet, a branching portion which is capable of branching the exhaustgases away from the nitrogen oxide trap; a switching feature for theexhaust gases in order to cause them to flow through the filteringportion or branching portion; and a three-way catalyzer which isarranged between the exhaust inlet and the upstream portion of thefiltering portion and the branching portion.
 2. The exhaust lineaccording to claim 1, wherein the switching feature blocks the filteringportion immediately downstream of the nitrogen oxide trap.
 3. Theexhaust line according to claim 1, wherein the switching featurecomprises a three-way valve which connects in parallel the branchingportion and the filtering portion to the remainder of the line.
 4. Theexhaust line according to claim 3, including a fork which is arrangedbetween the exhaust inlet and, on the one hand, the upstream portion ofthe filtering portion and, on the other hand, the upstream portion ofthe branching portion, and the three-way valve is arranged downstream ofthe filtering portion and the branching portion and is capable ofselectively connecting them to a common downstream portion of theexhaust line.
 5. The exhaust line according to claim 1, including atleast one sensor selected from the group comprising a sensor for sensingthe position of at least one valve, a temperature sensor and a gascomposition sensor and a diagnostic component for diagnosing theoperating state of the switching feature based on the or each sensor. 6.The exhaust line according to claim 1, including control for controllingthe switching feature and for acquiring the outlet temperature of theexhaust gases from the engine, and wherein the switching feature iscapable of generally branching the gases off towards the branchingportion when the temperature of the gases is greater than 600° C.
 7. Theexhaust line according to claim 1, including a control for controllingthe switching feature and for acquiring stoichiometric operatingconditions of the engine, and wherein the switching feature is capableof generally branching the gases off towards the branching portion whenthe engine operates in a state which is superstoichiometric in terms offuel.
 8. The exhaust line according to claim 6, wherein the control iscapable of periodically controlling the switching feature to temporarilycause the exhaust gases to flow through the filtering portion during thephases in which the exhaust gases generally flow through the branchingportion.
 9. The exhaust line according to claim 1, including a controlfor controlling the switching feature and for acquiring the futureoccurrence of a phase for operating the engine with a lean mixture whenthe engine operates with a rich mixture, and wherein that the control iscapable of controlling the switching feature to branch the gases offtowards the filtering portion when such an operating phase of the enginewith a lean mixture is detected.
 10. A propulsion assembly comprising:an engine which generally operates with a lean mixture; and an exhaustline comprising between an exhaust inlet and an exhaust outlet, anitrogen oxide trap which is fitted on a filtering portion; between thenitrogen oxide trap and the exhaust inlet, a branching portion which iscapable of branching the exhaust gases away from the nitrogen oxidetrap; a switching feature for the exhaust gases in order to cause themto flow through the filtering portion or branching portion; and athree-way catalyzer which is arranged between the exhaust inlet and theupstream portion of the filtering portion and the branching portion.